(1) Field of the Invention
The present invention relates to an incandescent bulb and an incandescent bulb filament, and particularly to an incandescent bulb filament of ribbon shape and an incandescent bulb using the filament.
(2) Description of the Related Art
A standard incandescent bulb includes a filament made of a conductive material, a bulb which envelops the filament, and a noble gas which is filled in the bulb, and has a high color rendering property. This incandescent bulb is widely used because it can be lighted using simple fixtures without using a lighting circuit such as a ballast, differently from a discharge lamp, and because it has a long history of use. (See, for example, Japanese Laid-Open Patent Application No. 3-102701 (Patent Document 1) and Japanese Laid-Open Patent Application No. 6-349358 (Patent Document 2)).
A filament is normally a coil made of a conductive wire, but Patent Document 2 discloses a filament made of a conductive ribbon. FIG. 14 shows a cross-sectional view of a filament 50 disclosed in Patent Document 2, and FIG. 15 shows a front view of a light-emitting element 50′ disclosed in Patent Document 2, and FIG. 16 shows a cross-sectional view of the light-emitting element 50′. As shown in FIG. 14, the filament 50 is comprised of a molded element 52 made of a 250 μm-wide conductive ribbon, and the molded element 52 is comprised of a series of elements 52a to 52k which are alternately arranged in parallel with spaces 53a to 53I of integral multiples of the conductive ribbon width. It should be noted that the spaces 53d and 53i are larger than other spaces 53a to 53c, 53e to 53h, 53j and 53k. As shown in FIG. 15 and FIG. 16, the light-emitting element 50′ is formed by making turns of the molded element 52 at connecting portions 54a and 54b. The front view of this light-emitting element 50′ in FIG. 15 shows as if there is no space between the elements 52a to 52k of the molded element 52 because they are arranged so as to overlap above and below one another. However, the cross-sectional view of the light-emitting element 50′ in FIG. 16 shows that an about 1 mm-wide space 55 is provided. The surface itself of the light-emitting element 50′ is the light-emitting surface which is comprised of a series of elements 52a to 52k. 
However, a standard incandescent bulb radiates electromagnetic waves including about 90 percent of infrared radiation and only about 10 percent of visible light. Such an incandescent bulb has low lamp efficiency of only about 13 Im/W, which poses a challenge to improvement in lamp efficiency. Here, lamp efficiency is defined as a ratio of an amount of light (luminous flux) to total energy generated per watt of power consumed by a lamp). Luminous flux is a measure of an amount of visible light propagated per unit time and is evaluated based on the sensitivity of a standard observer to the brightness of the light. Therefore, higher lamp efficiency means a larger amount of light generated per watt of consumed power, which provides energy savings.
A reflex lamp is a bulb having the inner surface, a part of which is coated with a reflective film. Visible light emitted from the filament in the backward direction of the lamp is reflected by the reflective film in the forward direction thereof so as to provide a higher illumination ahead of the lamp and has a higher lamp efficiency for the space which requires more brightness. However, even such a reflex lamp has far lower lamp efficiency than a fluorescent lamp, and therefore its lamp efficiency needs to be improved more.
In the light emitting element 50′ described in Patent Document 2, each of the spaces 53a to 53l has the width of 500 μm that is the integral multiple of the width (250 μm) of the ribbon which forms the light-emitting element 50′. Each of the spaces 55 provided in the light-emitting element 50′ has the width of 1 mm. In other words, there is a large space between the elements. Since a large space between the elements causes convection of a noble gas between the elements, a part of the heat generated in the light-emitting element 50′ is lost. Therefore, the temperature of the surface of the light-emitting element 50′ is not maintained constant across the board.
Generally speaking, envelopment of all over the surface of the filament (light-emitting element in Patent Document 2) by the heat generated in the filament allows the surface temperature to be maintained constant, and a bulb including such a filament has high lamp efficiency. In other words, a bulb including the light-emitting element 50′ which does not allow its surface temperature to be maintained constant has low lamp efficiency. Therefore, it is also necessary to improve the lamp efficiency of the bulb including the light-emitting element 50′ described in Patent Document 2.